The present disclosure generally relates to substrate processing, structure and fabrication of an optical head for optical storage.
An optical storage system uses an optical head to couple optical energy to and from a storage medium for retrieving or recording data. The optical head may use one or more lenses to tightly focus an optical beam so that the beam projects a small spot on the storage medium to increase the areal storage density. One embodiment of the optical head uses a lens that has a spherical surface and a flat surface opposing the spherical surface. The lens is engaged to a void in a substrate by, e.g., bonding, so that the flat surface of the lens is substantially in the same plane with a surface of the substrate. The lens and the substrate are generally made of different materials.
Such an optical head may be used in an optical storage system in a near-field configuration where the optical coupling between the optical head and the storage medium is achieved at least in part by evanescent fields and may also include coupling via propagation of light. The flat surface of the lens interfaces with the storage medium to provide the evanescent coupling. The optical head can be positioned over the storage medium by an air gap less than one wavelength via an air bearing.
The present techniques are in part based on the recognition that the material difference in the lens and the substrate can lead to surface variations due to a number of causes. For example, different materials for the lens and the substrate generally have different stresses of both the compressional and tensile types. This difference in the stresses may cause distortion in the flat surface of the lens or areas of the substrate adjacent to the lens. This distortion may be further exacerbated by environmental changes in the system such as the operating temperature since the lens and the substrate may have different thermal expansion coefficients. In addition, because of the difference in the mechanical properties such as hardness and elastic constants of the different materials, the materials can respond differently to machining processes such as lapping during manufacturing of the head. This may also result in surface variations on the substrate.
Surface variation on the substrate is undesirable because the strength of the optical coupling between the optical head and the storage medium, in the near-field configuration, is a function of the air gap. This is because the optical energy can be at least partly coupled by evanescent fields. Because the surface variation changes the thickness of the air gap, the coupled optical signals also changes accordingly. Such surface variation may exceed a predetermined tolerance limit, e.g., 0.15 micro inches in some near-field optical disk drives so that the variation in the optical signals is unacceptable. In addition, the optical head may include a transparent mesa on the flat surface of the lens. The mesa may be used for placing a coil to produce a desired magnetic field in a magneto-optic drive and to couple optical energy. The surface variation can lead to a variation in the height of the mesa and therefore cause undesired signal variations.
An optical head of the present disclosure includes an additional layer on the substrate that is formed of a material that matches one or more material properties of the lens material. In one embodiment, the substrate has first and second surfaces to define a through hole which forms an optical path and a lens with a spherical surface and a flat surface opposing the spherical surface is disposed in the optical path in the through hole. The flat surface is coplanar with a flat portion of the second surface. The optical head also includes a transparent layer contiguously formed over both the flat surface of the lens and the second surface of the substrate to form a monolithic layer. The refractive index of the additional layer may match that of the lens. In addition, the additional layer may have a material hardness greater than that of the lens to protect the lens and to reduce wear.
Another embodiment of the optical head includes a substrate having first and second surfaces to define a through hole which forms an optical path, a lens positioned in the optical path and partially located within the through hole at the second surface, and an additional layer formed over the second surface of the substrate. The lens has a spherical surface and a flat surface opposing the spherical -surface and is so disposed in the through hole that the flat surface protrudes beyond said second surface. The additional layer has an opening that conforms with a portion of the lens at the second surface, and is coplanar with the flat surface of said lens to form an interfacing surface.
These and other aspects and fabrication of the optical heads will become more apparent in light of the following detailed description, the accompanying drawings, and the appended claims.